The linked reference recommends erring on the side of precaution because the correlation of the observed record (per the WAIS Divide ice core) of temperature and snowfall for West Antarctica is more variable than indicated by current climate models (as normally it is assumed that as GMST increases so will the snowfall in West Antarctica, but the observed record indicates that that assumption may not be correct):

Abstract: "The Antarctic contribution to sea level is a balance between ice loss along the margin and accumulation in the interior. Accumulation records for the past few decades are noisy, and show inconsistent relationships with temperature. We investigate the relationship between accumulation and temperature for the past 31 ka using high-resolution records from the WAIS Divide ice core in West Antarctica. Although the glacial-interglacial increases result in high correlation and moderate sensitivity for the full record, the relationship shows considerable variability through time with high correlation and high sensitivity for the 0-8 ka period but no correlation for the 8-15 ka period. This contrasts with a general circulation model simulation which shows homogeneous sensitivities between temperature and accumulation across the entire time period. These results suggest that variations in atmospheric circulation are an important driver of Antarctic accumulation but they are not adequately captured in model simulations. Model-based projections of future Antarctic accumulation, and its impact on sea level, should be treated with caution."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

With a hat-tip to Greenbelt's post in the "Sea Level Rise" thread, the linked article discusses how NOAA has new information indicating that sea level could rise by 3m in the 2050-2060 timeframe due to instabilities in the WAIS:

Extract: "Think sea level rise will be moderate and something we can all plan for? Think again.Sea levels could rise by much more than originally anticipated, and much faster, according to new data being collected by scientists studying the melting West Antarctic ice sheet – a massive sheet the size of Mexico.That revelation was made by an official with the National Oceanic and Atmospheric Administration on Tuesday at the annual RIMS conference for risk management and insurance professionals in San Diego, Calif.The conference is being attended by more than 10,000 people, according to organizers. It was day No. 3 of the conference, which ends Wednesday.Margaret Davidson, NOAA’s senior advisor for coastal inundation and resilience science and services, and Michael Angelina, executive director of the Academy of Risk Management and Insurance, offered their take on climate change data in a conference session titled “Environmental Intelligence: Quantifying the Risks of Climate Change.”Davidson said recent data that has been collected but has yet to be made official indicates sea levels could rise by roughly 3 meters or 9 feet by 2050-2060, far higher and quicker than current projections. Until now most projections have warned of sea level rise of up to 4 feet by 2100.These new findings will likely be released in the latest sets of reports on climate change due out in the next few years.“The latest field data out of West Antarctic is kind of an OMG thing,” she said.Davidson’s purpose was to talk about how NOAA is sharing information with the insurance community and the public, and to explain how data on climate change is being collected.She explained that reports like those from the Intergovernmental Panel on Climate Change and the National Climate Assessment, which come out roughly every five years, are going on old data.By the time the scientists compiling those reports get the data it’s roughly two years old, because it took those gathering the data that long to collect it. It takes authors of the reports a few years to compile them.“By the time we get out the report, it’s actually synthesizing data from about a decade ago,” she said.Angelina’s focus was also on the data. He spoke about the ongoing development of the Actuaries Climate Index and the Actuaries Climate Risk Index.The goals of the projects are to create climate change indices that reflect an actuarial perspective, to create an index that measures changes in climate extremes, use indices to inform the insurance industry and the public, and promote the actuarial profession by contributing statistically to the climate change debate.So far their findings show the climate is definitely changing – though neither Davidson nor Angelina addressed the cause of this change, which they said was not the purpose of their talk.Angelina said a new way of looking at weather is required when dealing with climate change, and that just looking at averages isn’t enough to give an accurate picture of climate change and the risk it presents.The projects he’s involved with have instead looked at weather extremes.“By looking at extremes I can actually acknowledge that I have a problem,” he said."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

Technische Universitaet Dresden provides the following very useful website that documents Antarctic Ice Sheet mass change per the GRACE satellite, as updated every month. The first image shows the spatial distribution of ice mass loss (from 2002-2008 thru Jan 2016) across Antarctica. The second image provides a key to the GRACE mass change basins, from which it can be seen that for the WAIS ice mass loss comes primarily from basins 20, 21, 22 and 23 (which will be detailed in my next post).

Extract: "The Antarctic ice sheet, with a thickness of up to 4800 meter, has lost mass in the recent years. This was confirmed by a variety of scientific studies. Scientists now visualize the ice-mass loss: The interested public and scientific community can follow the Antarctic ice-mass changes month by month and divided by regions."

The four attached plots from Dresden detail the cumulative (from August 16 2002 to Jan 16 2016) ice mass loss from the AIS basins 20, 21, 22 and 23. These are all basins to watch to see whether ice mass loss from these areas increase non-linearly as/when GMST departures exceed 2C above pre-industrial:

The linked reference provides new field observations about changes in ocean circulation patterns from the continental shelf break to the coast in the Bellingshausen Sea, and they emphasize the importance of better understanding such recent changes in ocean circulation patterns in order to better understand the stability of the WAIS:

Abstract: "West Antarctic ice shelves have thinned dramatically over recent decades. Oceanographic measurements that explore connections between offshore warming and transport across a continental shelf with variable bathymetry towards ice shelves are needed to constrain future changes in melt rates. Six years of seal-acquired observations provide extensive hydrographic coverage in the Bellingshausen Sea, where ship-based measurements are scarce. Warm but modified Circumpolar Deep Water floods the shelf and establishes a cyclonic circulation within the Belgica Trough with flow extending towards the coast along the eastern boundaries and returning to the shelf break along western boundaries. These boundary currents are the primary water mass pathways that carry heat towards the coast and advect ice shelf meltwater offshore. The modified Circumpolar Deep Water and meltwater mixtures shoal and thin as they approach the continental slope before flowing westward at the shelf break, suggesting the presence of the Antarctic Slope Current. Constraining meltwater pathways is a key step in monitoring the stability of the West Antarctic Ice Sheet."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked (open access) reference cites research on four decades of marine glacier grounding line retreat in the Bellingshausen margin (see attached image). This region contributes significantly to the instability of the WAIS:

Abstract: "Changes to the grounding line, where grounded ice starts to float, can be used as a remotely-sensed measure of ice-sheet susceptibility to ocean-forced dynamic thinning. Constraining this susceptibility is vital for predicting Antarctica's contribution to rising sea levels. We use Landsat imagery to monitor grounding line movement over four decades along the Bellingshausen margin of West Antarctica, an area little monitored despite potential for future ice losses. We show that ~65% of the grounding line retreated from 1990-2015, with pervasive and accelerating retreat in regions of fast ice flow and/or thinning ice shelves. Venable Ice Shelf confounds expectations in that despite extensive thinning, its grounding line has undergone negligible retreat. We present evidence that the ice shelf is currently pinned to a sub-ice topographic high which, if breached, could facilitate ice retreat into a significant inland basin, analogous to nearby Pine Island Glacier."

See also:http://www.climatecentral.org/news/overlooked-area-antarctica-major-ice-loss-20408Extract: "During a 2009-2010 field mission, Bingham looked to shed more light on the region by scanning the ground below one of the fastest-moving Bellingshausen glaciers, the Ferrigno Ice Stream. He found a huge canyon underneath that is likely funneling warm ocean water under the ice.“This only served to highlight to me that there is so much about the Bellingshausen Sea sector of West Antarctica that has gone unmonitored while most of the world's eyes (glaciologically speaking) were looking beyond to Pine Island Glacier,” Bingham said.To get a better picture of the overall ice loss in the area, Bingham and his Ph.D. student Frazer Christie, analyzed hundreds of satellite images of the area going back to 1975 and tracked the position of the grounding line along 1,240 miles of coast.They found that 65 percent of the coastline had seen grounding line retreat since 1990, while only 7 percent had seen an advance. The total amount of ice lost over the last 40 years is about 390 square miles, an area about the size of Dallas.The results “show that this whole coastline has been in a state of retreat since records began in the early 1970s,” Bingham said. That contrasts with previous thinking that only certain glaciers, like the Ferrigno Ice Stream, were seeing significant ice loss while the rest were fairly stable.“The study illustrates that Antarctica is not immune to changes and that some of what we are seeing today started decades ago,” Eric Rignot, a NASA glaciologist who was not involved with the work, said in an email.One notable oddity was the Venable Ice Shelf, which has thinned, but hasn’t retreated much. The researchers think it is pinned to a ridge on the seafloor that is keeping it stable for now. Scientists think the same was true of Pine Island Glacier — for a while.Exactly what the pervasive ice loss in the Bellingshausen Sea area means for future global sea level rise isn’t entirely clear, in part because of the paucity of data from the area. Bingham says that researchers need a better idea of the topography underlying the ice so they can better model how it will change in the future."

The linked document describes a plan for the next decade of research in the WAIS to try to better quantify the rate and volume of change of ice mass loss now & in the future. While the document has many stellar authors, to me it is conspicuous that Eric Rignot is missing:

Extract: "This document is the outcome of a community science meeting held September 16-19, 2015 in Loveland Colorado, and a dedicated workshop on January 13-15, 2016 at the University of Colorado in Boulder.…

The primary geographic focus of the How Much, How Fast? effort will be the Thwaites Glacier and the adjacent areas of the Amundsen Sea."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked article is entitled: "Collapse of West Antarctic Ice Sheet Reveals Inadequacy of Current Climate Strategies", and provides a nice summary of key issues about a possible WAIS collapse this century.

The linked article is entitled: "Antarctica Past Points to Sea Level Threat", & the reference research clearly increases the recognized probability that the WAIS might collapse this century (with continued warming).

Interesting. You can see how this process drives tectonics. The ice mass loss at the periphery causes depressurisation of the puddle of fluid water and carbonate rich magma that the continental fringes float on due to seafloor sediment subductions. This causes trench blockwise subsidence for example as per the 700km / 800km long stretches some 50 km wide that dropped ~30 metres in the Valdivia May 22, 1960 9.5 and Offshore Maule/Biobío February 27, 2010 8.8 events off Chile. Simultaneously the increase in central Ice mass balance presurisses the ~500+ km deep superheated fluid basalt conduits that connect the continental keels to the midocean spreading zones. When you look at the repeat blockwise pattern of repeated ~50km wide food basalt sheets that spread out from the mid ocean trenches, and take note that the chemical composition of basalt formed is the same for thousands of km along the rifting zone, its clear that this is how it works. The seafloor is extruded in flood basalt pulses caused by the hydraulic pump of the ice sheet pistons, and blocks are simultaneously stacked under the continental fringes. Which is why the layers get younger as you go down. The heat, and tectonic mayhem released by these periodic isostatic overdrive episodes of course can cause large effect on sea levels and ice sheet stability

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Policy: The diversion of NZ aluminum production to build giant space-mirrors to melt the icecaps and destroy the foolish greed-worshiping cities of man. Thereby returning man to the sea, which he should never have left in the first place.https://en.wikipedia.org/wiki/McGillicuddy_Serious_Party

Abstract. Ice discharge from large ice sheets plays a direct role in determining rates of sea level rise. We map present-day Antarctic-wide surface velocities using Landsat 7 & 8 imagery spanning 2013–2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ~ 2008. The new mapping provides complete coastal and inland coverage of ice velocity with a mean error of < 10 m yr-1, resulting from multiple overlapping image pairs acquired during the daylit period. Using an optimized flux gate, ice discharge from Antarctica is 1932 ± 38 Gigatons per year (Gt yr-1) in 2015, an increase of 35 ± 15 Gt yr-1 from the time of the radar mapping. Flow accelerations across the grounding lines of West Antarctica's Amundsen Sea Embayment, Getz Ice Shelf and Marguerite Bay on the western Antarctic Peninsula, account for 89 % of this increase. In contrast, glaciers draining the East Antarctic Ice Sheet have been remarkably stable over the period of observation. Including modeled rates of snow accumulation and basal melt, the Antarctic ice sheet lost ice at an average rate of 186 ± 93 Gt yr-1 between 2008 and 2015. The modest increase in ice discharge over the past 7 years is contrasted by high rates of ice sheet mass loss and distinct spatial patters of elevation lowering. This suggests that the recent pattern of mass loss in Antarctica, dominated by the Amundsen Sea sector, is likely part of a longer-term phase of enhanced glacier flow initiated in the decades leading up to the first continent wide radar mapping of ice flow.

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

Extract: “In the iceberg-infested waters of the Amundsen Sea Embayment (ASE), it obtained the very first cores to be drilled from just in front of some of the mightiest glaciers on Earth.

Chief among these are Pine Island Glacier and Thwaites Glacier, colossal streams of ice that drain the White Continent and which are now spilling mass into the ocean at an alarming rate.

There's concern that deep, warm water is undercutting the glaciers, possibly tipping them into an unstoppable retreat. And that has global implications for significant sea-level rise. ...The goal was to retrieve seafloor sediments that would reveal the behaviour of the West Antarctic Ice Sheet (WAIS) in previous warm phases. To read the future in the past. …"If you find ice-rafted debris (stones dropped by icebergs), for example, you can be sure there was ice on land and that the ice had advanced to the coast," explained Claus-Dieter Hillenbrand from the British Antarctic Survey (BAS).

"But also new developments - especially what's known as geochemical provenance - have emerged in the last 10 years that mean it's even possible now to compare this material with rocks on land to pin down the actual sources in the hinterland."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

Extract: "So, is the eventual collapse of the West Antarctic Ice Sheet already inevitable? Model projections under low emissions scenarios suggest that ice sheet retreat could stabilise, but under medium and high scenarios, collapse is unstoppable.…The motto for early 21st Century cryospheric science should be “that happened faster than I thought it would.” Wherever we look, either in the past or in the present, we are challenged to keep up – in the ways we measure, theorise, project, and prepare for the future."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

Extract: "Scientists have documented a recent, massive melt event on the surface of highly vulnerable West Antarctica that, they fear, could be a harbinger of future events as the planet continues to warm.

In the Antarctic summer of 2016, the surface of the Ross Ice Shelf, the largest floating ice platform on Earth, developed a sheet of meltwater that lasted for as long as 15 days in some places. The total area affected by melt was 300,000 square miles, or larger than the state of Texas, the scientists report."

"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

What concerns me is that in a few decades time, austral summertime atmospheric river events may likely fall as rainfall instead of as snowfall. Such an atmospheric river event could devastate a local Antarctic ice shelf within a single week.

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked article cites a study that finds that the West Antarctic Rift System is the largest volcanic region on Earth, and that some of these volcanic may very well become more active should the WAIS thin sufficiently, which would be a positive feedback mechanism for a more complete collapse of the WAIS:

Title: "Scientists find what they think is largest volcanic region on Earth hidden in Antarctica after student's idea"

Extract: "A remote survey discovered 91 volcanoes ranging in height from 100m to 3,850m in a massive region known as the West Antarctic Rift System.

Geologists and ice experts say the range has similarities to east Africa's volcanic ridge, currently acknowledged to be the densest concentration of volcanoes in the world.

Researchers from the University of Edinburgh remotely surveyed the underside of the ice sheet for hidden peaks of basalt rock, like those of other volcanoes in the region whose tips push above the ice.…Previous studies have suggested that volcanic activity may have occurred in the region during warmer periods and could increase if Antarctica's ice thins in a warming climate.

Dr Robert Bingham, of the University of Edinburgh's School of GeoSciences, said: "It is fascinating to uncover an extensive range of volcanoes in this relatively unexplored continent.

"Better understanding of volcanic activity could shed light on their impact on Antarctica's ice in the past, present and future, and on other rift systems around the world.""

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

That Nicolas paper(DOI: 10.1038/ncomms15799 ) has a figure in it that tells me Mercer's indicator to watch midsummer 0C isotherm is flashing, it's in the deep interior of Ross shelf in 2016. I attach fig 1c

for the locations of the weather stations, see the (open access) article.

Abstract: "Over the past two decades the primary driver of mass loss from the West Antarctic Ice Sheet (WAIS) has been warm ocean water underneath coastal ice shelves, not a warmer atmosphere. Yet, surface melt occurs sporadically over low-lying areas of the WAIS and is not fully understood. Here we report on an episode of extensive and prolonged surface melting observed in the Ross Sea sector of the WAIS in January 2016. A comprehensive cloud and radiation experiment at the WAIS ice divide, downwind of the melt region, provided detailed insight into the physical processes at play during the event. The unusual extent and duration of the melting are linked to strong and sustained advection of warm marine air toward the area, likely favoured by the concurrent strong El Nino event. The increase in the number of extreme El Nino events projected for the twenty-first century could expose the WAIS to more frequent major melt events."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

Abstract: "The possibility that a deep mantle plume manifests Pliocene and Quaternary volcanism and potential elevated heat flux in West Antarctica has been studied for more than 30 years. Recent seismic images support the plume hypothesis as the cause of Marie Byrd Land (MBL) volcanism and geophysical structure. Mantle plumes may more than double the geothermal heat flux above nominal continental values. A dearth of in situ ice sheet basal data exists that samples the heat flux. Consequently, we examine a realistic distribution of heat flux associated with a possible late Cenozoic mantle plume in West Antarctica and explore its impact on thermal and melt conditions at the ice sheet base. We use a simple analytical mantle plume parameterization to produce geothermal heat flux at the base of the ice sheet. The three-dimensional ice flow model includes an enthalpy framework and full-Stokes stress balance. As both the putative plume location and extent are uncertain, we perform broadly scoped experiments to characterize the impact of the plume on geothermal heat flux and ice sheet basal conditions. The experiments show that mantle plumes have an important local impact on the ice sheet, with basal melting rates reaching several centimeters per year directly above the hotspot. In order to be consistent with observations of basal hydrology in MBL, the upper bound on the plume-derived geothermal heat flux is 150 mW/m2. In contrast, the active lake system of the lower part of Whillans Ice Stream suggests a widespread anomalous mantle heat flux, linked to a rift source."

See also the linked associated article entitle: "Study bolsters theory of heat source under Antarctica"

Extract: "A new NASA study adds evidence that a geothermal heat source called a mantle plume lies deep below Antarctica's Marie Byrd Land, explaining some of the melting that creates lakes and rivers under the ice sheet. Although the heat source isn't a new or increasing threat to the West Antarctic ice sheet, it may help explain why the ice sheet collapsed rapidly in an earlier era of rapid climate change, and why it is so unstable today.…The Marie Byrd Land mantle plume formed 50 to 110 million years ago, long before the West Antarctic ice sheet came into existence. At the end of the last ice age around 11,000 years ago, the ice sheet went through a period of rapid, sustained ice loss when changes in global weather patterns and rising sea levels pushed warm water closer to the ice sheet—just as is happening today. Seroussi and Ivins suggest the mantle plume could facilitate this kind of rapid loss."

Based on my interpretation of the two linked references, I suspect that local ice cliff failures near the base of the Thwaites Ice Tongue (see the four images) will begin sometime 2025 and 2033, and will be initiated due to influences from Super El Nino events in that timeframe:

Abstract. "Thwaites Glacier (TG), West Antarctica, has been losing mass and retreating rapidly in the past few decades. Here, we present a study of its calving dynamics combining a two-dimensional flow-band full-Stokes (FS) model of its viscous flow with linear elastic fracture mechanics (LEFM) theory to model crevasse propagation and ice fracturing. We compare the results with those obtained with the higher-order (HO) and the shallow-shelf approximation (SSA) models coupled with LEFM. We find that FS/LEFM produces surface and bottom crevasses that are consistent with the distribution of depth and width of surface and bottom crevasses observed by NASA’s Operation IceBridge radar depth sounder and laser altimeter, whereas HO/LEFM and SSA/LEFM do not generate crevasses that are consistent with observations. We attribute the difference to the nonhydrostatic condition of ice near the grounding line, which facilitates crevasse formation and is accounted for by the FS model but not by the HO or SSA models. We find that calving is enhanced when pre-existing surface crevasses are present, when the ice shelf is shortened or when the ice shelf front is undercut. The role of undercutting depends on the timescale of calving events. It is more prominent for glaciers with rapid calving rates than for glaciers with slow calving rates. Glaciers extending into a shorter ice shelf are more vulnerable to calving than glaciers developing a long ice shelf, especially as the ice front retreats close to the grounding line region, which leads to a positive feedback to calving events. We conclude that the FS/LEFM combination yields substantial improvements in capturing the stress field near the grounding line of a glacier for constraining crevasse formation and iceberg calving."

Extract: "In our simulations, we find that crevasses propagate significantly faster near the ice front when the ice shelf is shortened.…The reason for the propagation of crevasses is the existence of a nonhydrostatic condition of ice immediately downstream of the grounding line, which is not accounted for in simplified models that assume hydrostatic equilibrium everywhere on the ice shelf. We also find that calving is enhanced in the presence of pre-existing surface crevasses or shorter ice shelves or when the ice front is undercut. We conclude that it is important to consider the full stress regime of ice in the grounding line region to replicate the conditions conducive to calving events, especially the nonhydrostatic condition that is critical to propagate the crevasses."

&

The second linked reference confirms that the ENSO is directly associated with surface air temperatures across the interior of West Antarctica, and I note that the frequency of extreme El Nino events is projected to double when the global mean surface temp. anom. gets to 1.5C:

Kyle R. Clem, James A. Renwick, and James McGregor (2017), "Large-Scale Forcing of the Amundsen Sea Low and its Influence on Sea Ice and West Antarctic Temperature", Journal of Climate, https://doi.org/10.1175/JCLI-D-16-0891.1

Abstract: "Using empirical orthogonal function (EOF) analysis and atmospheric reanalyses, we examine the principal patterns of seasonal West Antarctic surface air temperature (SAT) and their connection to sea ice and the Amundsen Sea Low (ASL). During austral summer, the leading EOF (EOF1) explains 35% of West Antarctic SAT variability and consists of a widespread SAT anomaly over the continent linked to persistent sea ice concentration anomalies over the Ross and Amundsen Seas from the previous spring. Outside of summer, EOF1 (explaining ~40-50% of the variability) consists of an east-west dipole over the continent with SAT anomalies over the Antarctic Peninsula opposite those over western West Antarctica. The dipole is tied to variability in the Southern Annular Mode (SAM) and in-phase El Niño-Southern Oscillation (ENSO) / SAM combinations that influence the depth of the ASL over the central Amundsen Sea (near 105°W). The second EOF (EOF2) during autumn, winter, and spring (explaining ~15-20% of the variability) consists of a dipole shifted approximately 30 degrees west of EOF1 with a widespread SAT anomaly over the continent. During winter and spring, EOF2 is closely tied to variability in ENSO and a tropically-forced wavetrain that influences the ASL in the western Amundsen / eastern Ross Seas (near 135°W) with an opposite sign circulation anomaly over the Weddell Sea; the ENSO-related circulation brings anomalous thermal advection deep onto the continent. We conclude the ENSO-only circulation pattern is associated with SAT variability across interior West Antarctica, especially during winter and spring, while the SAM circulation pattern is associated with an SAT dipole over the continent."

If one accepts that a rough glacial bed on a marine-terminating glacier reflects a lack of sliding erosion, the roughness also marks a region where during previous retreats ice cliff collapse was the dominant mechanism - not sliding nor creep.

I'm no specialist in this area, but the descriptions I've read of the models don't seem to take such bed changes into account as the local singularities they likely are.

Apropos the third diagram above, further retreat by the Thwaites Glacier can be expected to be through ICI starting almost immediately.

So why, ASLR, do you expect a delay to 2025+ and the action of "Super El Nino events"? Are you expecting a collapse of the Thwaites eastern ice shelf first?

I was thinking the same thoughts about the bottom roughness being an indicator of past cliff failures in this area; however, the residual ice shelf in this area needs to retreat to where the red line meets the bed before the ice geometry is adequate to support cliff failures and also the mélange needs to clear out of the way. I picked 2025 as the earliest date for the next super el nino as at a ECS of 4.5C I estimate that we will reach a GMSTA of 1.5C circa 2024, and per the second reference in my last link that will drive the period for super el ninos from 20-years to 10-years (for from 2015/16 to 2025/26). The extra warm water from such a Super El Nino is needed to melt back the grounding line to meet the red line. Also, I do not think that the Thwaites Eastern Ice Shelf will become unpinned until around 2035 as the ice velocities are slower over there.

Best,ASLR

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

As a follow-on to my last post, I note that per the two attached versions of the same plot from Pollard, DeConto & Alley (2015), the impact of cliff failures without hydrofracturing is limited (beyond normal glacial sliding/flowing behavior). Thus as I do not believe that we will be approaching the GMSTA value of about 2.7C before about 2040 (assuming ECS =4.5C), I do not expect significant ice cliff failures along line A-B before about 2040.

The graph you show uses sea level rise on the y axis. Sea level rise seems irrelevant for this part of the discussion..

Your earlier image of the Thwaites profile indicates that by roughly the time the calving face retreats 20 km from the grounding line the cliff faces are high enough to spontaneously fail by their own weight -- no free water needed. So I do not see the need for hydrofracturing for a high speed of collapse, just a means of moving ice cubes out to sea.

The graph you show uses sea level rise on the y axis. Sea level rise seems irrelevant for this part of the discussion..

Your earlier image of the Thwaites profile indicates that by roughly the time the calving face retreats 20 km from the grounding line the cliff faces are high enough to spontaneously fail by their own weight -- no free water needed. So I do not see the need for hydrofracturing for a high speed of collapse, just a means of moving ice cubes out to sea.

Keep up the good work,Steve

The vertical axis of WAIS contribution to sea level rise, slr, is just a convenient unit for measuring net ice mass loss (ice discharge less snowfall). Nevertheless, it is an aggregate of all ice mass contributions to slr for the WAIS and does not necessarily apply to the case we are considering along line A-B; so the point that you make is valid. Still the hydrofracturing would clearly help to get the grounding line to retreat the almost 20km necessary for the cliff failures to occur; which was my main issue with citing the 2040 date.

Edit: Also, the graph/plot was for a recent interglacial (I think the Eemian, but I do not remember specifically), with less radiative forcing than where we are headed.

« Last Edit: December 02, 2017, 12:33:24 AM by AbruptSLR »

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

The first image shows the trough that transects a portion of the Thwaites ice plug, which has a downstream 'mouth' centered on about W107.1 by S75.5; that is ignored by Yu et al. (2017) 2D analysis along line A-B (See Reply #478). Also, note that the first image shows that the ice flow feeding both the Thwaites Eastern Ice Shelf and the Thwaites Ice Tongue are inclined from the axis of these two features and that a compression field in the ice near mouth of this trough, redirects this ice flow to align with those ice features. Also, the second image shows that the subglacial drainage system beneath Thwaites exists through this trough.

Now the linked reference studies a subglacial draining event beneath Thwaites Glacier from June 2013 to January 2014 that drained four subglacial lakes (see the third and fourth images), and increased the velocity of the ice flow for Thwaites by about 10% during this drainage event, and in this velocity increase was probably higher along the alignment of the trough due to the basal water flowing through the trough. Now higher ice velocities mean lower ice surface elevation (due to the conservation of ice discharge), which likely is pre-cracking the ice in this area; which, in my opinion makes it more likely to develop future ice cliffs, particularly as Yu et al. (2017) indicates that these subglacial lake drainage events can happen as frequently as every 20-years, which might means that the next such event might occur around 2033 to 2034, which might coincide with a Super El Nino event (see Reply #480)

Abstract. We present conventional and swath altimetry data from CryoSat-2, revealing a system of subglacial lakes that drained between June 2013 and January 2014 under the central part of Thwaites Glacier, West Antarctica (TWG). Much of the drainage happened in less than 6 months, with an apparent connection between three lakes spanning more than 130 km. Hydro-potential analysis of the glacier bed shows a large number of small closed basins that should trap water produced by subglacial melt, although the observed largescale motion of water suggests that water can sometimes locally move against the apparent potential gradient, at least during lake-drainage events. This shows that there are important limitations in the ability of hydro-potential maps to predict subglacial water flow. An interpretation based on a map of the melt rate suggests that lake drainages of this type should take place every 20–80 years, depending on the connectivity of the water flow at the bed. Although we observed an acceleration in the downstream part of TWG immediately before the start of the lake drainage, there is no clear connection between the drainage and any speed change of the glacier."

See also the article entitled: "Hidden lakes drain below West Antarctica’s Thwaites Glacier".

Extract: "Researchers at the University of Washington and the University of Edinburgh used data from the European Space Agency’s CryoSat-2 to identify a sudden drainage of large pools below Thwaites Glacier, one of two fast-moving glaciers at the edge of the ice sheet. The study published Feb. 8 in The Cryosphere finds four interconnected lakes drained in the eight months from June 2013 and January 2014. The glacier sped up by about 10 percent during that time, showing that the glacier’s long-term movement is fairly oblivious to trickles at its underside.…Melting at the ice sheet base would refill the lakes in 20 to 80 years, Smith said. Over time meltwater gradually collects in depressions in the bedrock. When the water reaches a certain level it breaches a weak point, then flows through channels in the ice. As Thwaites Glacier thins near the coast, its surface will become steeper, Smith said, and the difference in ice pressure between inland regions and the coast may push water coastward and cause more lakes to drain."

The second image show the consequences of an abrupt ice surface drop event that happened near the 'mouth' of the trough sometime between Jan 2012 and Jan 2013.

The third image shows the nature of the cracking/crevasse pattern in ice near the 'mouth' of the trough in January 2013 (after the event shown in the second image), together with the locations of the grounding line (green) and the calving front (orange).

The fourth image shows a Sentinel-1 image from Nov 27 2017, that should that cracks/crevasses are now occurring over the trough upstream of the 'mouth'.

These images indicate a trend that supports the idea that more cracking/crevasses will form in this region at the base of the Thwaites Ice Tongue; which may facilitate the formation of ice cliff failure mechanisms in this area beginning sometime from 2033 to 2040, when I suspect that hydrofracturing may be frequent in this area.

As a follow-on to my last two posts, I provide the following background/related images:

The first image from Stearns et al. (2008) shows a similar subglacial lake drainage event for the Byrd Glacier from Dec 2005 to Feb 2007, that also temporarily accelerated ice flow velocities (due to basal lubrication).

The second image how the relocation of the Amundsen Sea Low, ASL, during El Nino events can cause winds to direct more than typical amounts of warm CDW into the ASE.

The third image from Bakker et al. (2017) shows how a glacial cross-section very similar to the one along line A-B that Yu et al (2017) analyzed at the base of the Thwaites Ice Tongue, is believed to have behaved during the Eemian (MIS 5 peak) event; which shows the importance of the role of the warm CDW in triggering subsequent cliff failures.

The fourth image of cliff failures for the Jakobshavn Glacier (in Greenland) typically roll after calving so as to reduce the drafts of the calved icebergs, thus reducing the likelihood that the mélange will provide significant buttressing to any subsequent cliff failures.

Taken together, these images support the idea that cliff failures could progressively radiate out from the base of the Thwaites Ice Tongue after 2035 to 2040, particularly if the Thwaites Eastern Ice Shelf becomes unpinned circa 2035 as I suspect that it might.

As a follow-on to my last few posts, I provide the following related images:

The first image shows that during the combination of an El Nino event and negative SAM conditions, Rossby Waves form in the atmosphere that telecommunicate heat directly from the Tropical Pacific directly to the WAIS, where it promotes surface ice melt events (which promotes hydrofracture events).

The second image shows my marks in green (in 2013, in this thread) showing my guesses of zones of grounding line retreats in the WAIS circa 2040 where the green zone near the base of the Thwaites Ice Tongue have ice cliff faces, while all the other green zones indicate estimated grounding line retreats beneath ice shelves.

The third image shows Hansen et al. (2016) estimates of changes in both GMSTA and Earth Energy Imbalance, EEI, due to assumed major freshwater hosing events (such as the potential collapse of the WAIS), and where the orange curves roughly correspond to the timing indicated in the second image.

The linked reference discusses how gas hydrates in the bed sediment beneath marine glaciers can cause 'sticky spots' that can regulate ice stream flow rates. Also, during a potential abrupt collapse of the WAIS such methane hydrates in the seafloor could result in significant methane emissions into the atmosphere (acting as a positive feedback mechanism):

Extract: "Based on the presence of extensive sedimentary basins and modelling studies (Wadham et al., 2012; Wallmann et al., 2012) it is proposed that abundant gas hydrate accumulations are present beneath the ice sheets of Greenland and Antarctica. Also, gas hydrates have been identified in ice core samples obtained from above the subglacial Lake Vostok in East Antarctica (Uchida et al., 1994). The role of potentially widespread gas hydrate reservoirs in the modification of the thermomechanical regime at the base of contemporary ice sheets, which makes them critically sensitive, as well as their impact on ice steam force balance and dynamics has, so far, not been recognised. This control that was previously unforeseen, given the current lack of knowledge with regard to the distribution of gas hydrate, represents a significant unknown in attempts to model the current and future discharge and evolution of contemporary ice sheets, as well as their contribution to rising global sea levels."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked reference shows that ice mass loss from the WAIS is a nonlinear function of ocean forcing period. While many consensus scientists consider ocean forcing a slow response mechanism, reasons to suspect that such nonlinear behavior could be trigger in the next few decades include:1. The Southern Ocean has been warming since 1750, which is a long period of ocean forcing.2. The Antarctic Ozone Hole has been advecting warm CDW to the grounding line of key WAIS marine glaciers since the 1970's (which is a somewhat long period).3. Due to both the Antarctic Ozone Hole and Greenland Ice Sheet, GIS, ice mass loss, Agulhas Leakage has been documented to be occurring for years; which is increasing Arctic Amplification.4. The Beaufort Gyre has been increasingly accumulating freshwater for longer periods since the mid-20th century and has not generated a pulsed release of freshwater since 2004 (apparently due to ice mass loss from the GIS and associated changes in ocean currents). Furthermore, a sharp increase in ice mass loss from Jakobshaven Glacier between 2018 and 2028 could cause the Beaufort Gyre to accumulate several times its typical quantity of freshwater; which might then be released in a large pulse in to the North Atlantic thus slow the AMOC and warming Antarctic (including advecting more warm CDW to key marine glacier grounding lines and ice shelves) via the bipolar seesaw mechanism.

Abstract: "West Antarctic Ice Sheet loss is a significant contributor to sea level rise. While the ice loss is thought to be triggered by fluctuations in oceanic heat at the ice shelf bases, ice sheet response to ocean variability remains poorly understood. Using a synchronously coupled ice-ocean model permitting grounding line migration, this study evaluates the response of an ice sheet to periodic variations in ocean forcing. Resulting oscillations in grounded ice volume amplitude is shown to grow as a nonlinear function of ocean forcing period. This implies that slower oscillations in climatic forcing are disproportionately important to ice sheets. The ice shelf residence time offers a critical time scale, above which the ice response amplitude is a linear function of ocean forcing period and below which it is quadratic. These results highlight the sensitivity of West Antarctic ice streams to perturbations in heat fluxes occurring at decadal time scales."

« Last Edit: December 18, 2017, 02:27:09 AM by AbruptSLR »

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked reference adds new information about the telecommunication of tropical energy from the Equatorial Pacific to West Antarctica (the image shows a pattern of atmospheric Rossby wave train from the Nino 3 area to West Antarctica), where this energy can contribute to ice mass loss from the WAIS:

Abstract: "Occupying about 14% of the world’s surface, the Southern Ocean plays a fundamental role in ocean and atmosphere circulation, carbon cycling and Antarctic ice-sheet dynamics. Unfortunately, high interannual variability and a dearth of instrumental observations before the 1950s limits our understanding of how marine–atmosphere–ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing. Here we integrate climate-sensitive tree growth with ocean and atmospheric observations on southwest Pacific subantarctic islands that lie at the boundary of polar and subtropical climates (52–54 degrees S). Our annually resolved temperature reconstruction captures regional change since the 1870s and demonstrates a significant increase in variability from the 1940s, a phenomenon predating the observational record. Climate reanalysis and modelling show a parallel change in tropical Pacific sea surface temperatures that generate an atmospheric Rossby wave train which propagates across a large part of the Southern Hemisphere during the austral spring and summer. Our results suggest that modern observed high interannual variability was established across the mid-twentieth century, and that the influence of contemporary equatorial Pacific temperatures may now be a permanent feature across the mid- to high latitudes."

I am too cheap to access the linked references, but I cite them as their summaries indicate that two of the factors (the rate of ocean warming at depth & the rate of loss of buttressing from ice shelves) driving ice mass loss from Antarctic marine glaciers have been previously underestimated:

The following reference discusses how strengthening westerly winds over the Southern Ocean is increasing the volume of warm water at a depth near 500m.

76A2568Ice sheets and sea level: to rise, or not to rise, that is no longer the questionSophie NowickiCorresponding author: Sophie NowickiCorresponding author e-mail: sophie.nowicki@nasa.govOn 18 April 2017, the New York Times published an article entitled ‘When Rising Seas Transform Risk into Certainty’, illustrating that rising sea level is now in the public eye. When thinking about sea level, the question is no longer whether levels are rising, but how to increase confidence in projections of ice-sheet evolution and reduce the uncertainty in projection of sea level. These questions lie at the heart of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), the first time that an effort dedicated to ice sheets is part of the climate model endeavor (CMIP) that forms the foundation of the IPCC reports. This presentation will introduce ISMIP6, review progress made in both the ice sheet and climate modeling communities in order to improve our understanding of how ice sheets contribute to the climate system, before exploring the challenges ahead and how anyone with an interest in the polar regions can contribute to this new venture.

76A2573Coupled ice shelf–ocean modelling and complex grounding line retreat for Pine Island GlacierJan de Rydt, Hilmar GudmundssonCorresponding author: Jan De RydtCorresponding author e-mail: janryd69@bas.ac.ukRecent observations and modelling work have shown a complex mechanical coupling between Antarctica’s floating ice shelves and the adjacent grounded ice sheet. A prime example is Pine Island Glacier, West Antarctica, which has a strong negative mass balance caused by a recent increase in ocean-induced melting of its ice shelf. The mass loss coincided with the retreat of its grounding line from a seabed ridge on which it was at least partly grounded until the 1970s. At present, it is unclear what caused the onset of this retreat, and how feedback mechanisms between the ocean and ice-shelf geometry have influenced the ice dynamics. To address these questions, we present results from an offline coupling between a state-of-the-art shallow-ice flow model with grounding-line resolving capabilities, and a three-dimensional ocean general-circulation model with a static implementation of the ice shelf. We simulate the retreat from an idealized seabed ridge in response to changes in the ocean forcing, and show that the retreat becomes irreversible after 20 years of warm ocean conditions. A comparison with experiments with a simple depth-dependent meltrate parameterization demonstrates that such parameterizations are unable to capture the details of the retreat process, and they overestimate mass loss by more than 40% over a 50-year timescale. In a second set of experiments, we used the coupled model to simulate the evolution of all Amundsen Sea glaciers under a range of warm and cold ocean scenarios.

76A2592West Antarctic surface elevation change from CryoSat-2 radar altimetry and multi-mission lidar mappingTyler Sutterley, Isabella Velicogna, Eric Rignot, Jeremie Mouginot, Thorsten Markus, Tom NeumannCorresponding author: Tyler SutterleyCorresponding author e-mail: tyler.c.sutterley@nasa.govWe present estimates of surface elevation change at the Bellinghausen Sea, Amundsen Sea and Getz regions of the West Antarctic Ice Sheet (WAIS) from CryoSat-2 radar altimetry measurements and a combination of satellite and airborne laser altimetry measurements. These regions are currently some of the most responsible for sea-level rise from the Antarctic continent. Our radar altimetry method combines Level-2 elevation measurements from the low-resolution mode (LRM) and the interferometric synthetic aperture mode (SARin) of the synthetic aperture interferometric radar altimeter (SIRAL) ranging instrument. Our laser altimetry method combines measurements from the Airborne Topographic Mapper (ATM), the Land, Vegetation and Ice Sensor (LVIS) and the Ice Cloud and land Elevation Satellite (ICESat-1). The laser altimetry method allows us to extend the records of each instrument, increases the overall spatial coverage compared to a single instrument, and produces high-quality, coherent maps of surface-elevation change. We compare elevation-change measurements for major outlet glaciers in West Antarctica to assess the regional stability. We find CryoSat-2 and laser altimetry estimates produce comparable rates of elevation change in regions with lower surface slopes. The agreement is lower in regions with mountainous terrain and small outlet glaciers.

76A2595Land ice in version 2.0 of the Community Earth System ModelWilliam Lipscomb, Jeremy Fyke, Gunter Leguy, Jan Lenaerts, William Sacks, Leo van Kampenhout, Miren VizcainoCorresponding author: William LipscombCorresponding author e-mail: lipscomb@ucar.eduThe summer 2017 release of the Community Earth System Model version 2 (CESM2) includes major advances, compared to CESM1, in the treatment of ice sheets and their interactions with the climate. The dynamic ice sheet model is version 2.1 of the Community Ice Sheet Model (CISM2.1), which has a higher-order velocity solver (suitable for simulating fast flow in ice streams and ice shelves) and improved treatments of basal and calving physics. In long spin-ups for the initMIP-Greenland project, the modeled Greenland ice extent, volume and surface velocity agree well with observations. In coupled runs, the Community Land Model (CLM) computes the ice-sheet surface mass balance (SMB) in multiple elevation classes, and the coupler downscales the SMB to the fine-scale CISM grid. Recent CLM snow physics improvements give a more realistic SMB for both Greenland and Antarctica. CESM2 supports two-way coupling of ice sheets, with ice-sheet evolution feeding back conservatively on land-surface types and elevation. A developmental version of CISM, which includes a grounding-line parameterization and an ocean plume model, has been verified for marine ice-sheet benchmark experiments and has the potential to be used for dynamic Antarctic simulations in future versions of CESM.

76A2598Characterizing uncertainty in projected changes of Antarctic surface temperature, precipitation and sea ice extentDavid SchneiderCorresponding author: David SchneiderCorresponding author e-mail: dschneid@ucar.eduSome of the largest uncertainties in projected anthropogenic climate change impacts occur in or are linked to Antarctica and the Southern Ocean. Projected changes in Antarctic surface mass balance, sea-ce extent and surface temperature differ widely among current-generation climate models, and this uncertainty likely has roots in the mean states (climatologies) of the models. In this presentation, we will highlight projected changes in surface air temperature and precipitation over the Antarctic Ice Sheet and relate the magnitude of these changes to the initial climatology of the model. To characterize the roles of natural variability and model (structural) uncertainty in the spread of these projections, we will use output from the Community Earth System Model Large Ensemble as well as the CMIP5 archive.

76A2599Observations of recent climate change in East Antarctica outpace future model simulationsBrooke Medley, Joseph McConnell, Thomas Neumann, Carleen H. Reijmer, Sepp Kipfstuhl, Michael SiglCorresponding author: Brooke MedleyCorresponding author e-mail: brooke.c.medley@nasa.govThe West Antarctic Ice Sheet (WAIS) is experiencing rapid warming and substantial ice-mass loss, designating it as one of the regions most vulnerable to change in Antarctica, especially in comparison to the more massive East Antarctic Ice Sheet (EAIS), which is thought to be undergoing little or no mass change. Thus, researchers consider the high, dry EAIS stable with little warming and no significant change in snowfall since 1957. Here, we present new observations of snow accumulation and air temperature near Kohnen station in Queen Maud Land that suggest that this region can experience climate change at a pace similar to or potentially more rapid than observations from WAIS. Over the past 75 years, snow accumulation has increased 5.2 ± 3.7% per decade, a rate that is 1.5 times more rapid than any 75-year interval in the previous nearly 2000 years (1–1850 CE). The recent 20-year mean annual accumulation is 16.5 mm w.e. larger than the preindustrial mean of 66.2 mm w.e. Similarly, annual air temperature has been increasing by 1.1 ± 0.7 °C per decade since 1998, with significant seasonal increases in autumn and spring. By comparing our observed changes with output from the Community Earth System Model, we find that the observed rates of accumulation and temperature change outpace the model simulations by several decades even under the high-emission RCP8.5 scenario.

76A2619Effects of climate variability on marine ice-sheet stabilityMatthew Hoffman, Jeremy Fyke, Stephen PriceCorresponding author: Matthew HoffmanCorresponding author e-mail: mhoffman@lanl.govTheory, modeling, and observations indicate that marine ice sheets on a retrograde bed are only conditionally stable. Previous modeling studies have shown that rapid, unstable retreat can occur when steady ice-shelf basal melting causes the grounding line to retreat past restraining bedrock bumps. Here we explore the initiation and evolution of unstable retreat when the ice-shelf basal melt forcing includes temporal variability mimicking realistic climate variability. We use the three-dimensional, higher-order Model for Prediction Across Scales-Land Ice (MPASLI) model in an idealized model configuration similar to Pine Island Glacier. We find that climate variability has a complex relationship to marine ice-sheet stability and can delay or accelerate unstable retreat.

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

The linked article discusses how the ENSO cycle impacts on Antarctic Pacific Sector ice shelves. Thus with the frequency of extreme El Nino events predicted to increase with global warming, we can expect the Antarctic Pacific Sector ice shelves to degrade more rapidly in the coming decades:

Extract: "El Niño events are known for bringing floods to South America and contributing to wildfires in Indonesia, but new research reveals they also affect the height and mass of ice shelves in Antarctica.…During an El Niño event, many of the ice shelves around West Antarctica receive more snow on their surface, but also lose more ice from underneath because of warm ocean water.Overall, the ice shelves actually lose mass during an El Niño, the research finds, making such events an important factor in the year-to-year fluctuations of ice shelf size.With more “extreme” El Niño events expected as global temperatures rise, West Antarctica’s ice shelves could see larger fluctuations in height and mass, …"

The linked reference helps to quantify inter-decadal trends of ice mass loss in Marie Byrd Land. Maybe as El Nino's intensify with continued warming, these climateocean trends will become more significant:

Abstract. Over the past 20 years satellite remote sensing has captured significant downwasting of glaciers that drain the West Antarctic Ice Sheet into the ocean, particularly across the Amundsen Sea Sector. Along the neighbouring Marie Byrd Land Sector, situated west of Thwaites Glacier to Ross Ice Shelf, glaciological change has been only sparsely monitored. Here, we use optical satellite imagery to track grounding-line migration along the Marie Byrd Land Sector between 2003 and 2015, and compare observed changes with ICESat and CryoSat- 2-derived surface elevation and thickness change records. During the observational period, 33 % of the grounding line underwent retreat. The greatest retreat rates were observed along the 650-km-long Getz Ice Shelf, further west of which only minor retreat occurred. The relative glaciological stability west of Getz Ice Shelf can be attributed to a divergence of the Antarctic Circumpolar Current from the continental-shelf break at 135° W, coincident with a transition in the morphology of the continental shelf. Along Getz Ice Shelf, grounding-line retreat reduced by 68 % during the CryoSat-2 era relative to earlier observations. This slowdown is a likely response to reduced oceanic forcing, as inferred from climate reanalysis data. Collectively, our findings underscore the importance of spatial and inter-decadal variability in climate and ocean interactions in moderating glaciological change around Antarctica.

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

We should all remember that if the WAIS collapses, both during and after the collapse, the associated local tidal elevation ranges and currents will be significantly different than those assessed in the linked reference:

Abstract: "Ocean tides are the main source of high-frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide-induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite-based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice-ocean models and analytical glacier models that quantify the effect of ocean tides on lower-frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice-ocean interface, and higher-resolution models with improved representation of tidal energy sinks."

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“It is not the strongest or the most intelligent who will survive but those who can best manage change.” ― Leon C. Megginson

As I am quite new here I wonder whether a table exists, which shows the retreat of the grounding lines of Thwaites, Kohler, Smith, Pope and Pine Island Glacier in the last 20 years or so in km or miles as a function of time. In addition it would be intersting to know whether a change in grounding line retreat velocity is due to local effects (e.g. bumps in the glacier's way) or general conditions (e.g. warming of the deeper ocean water) and how / whether there is a correlation between the speeding-up of each glacier's flow downstream and the grounding line position. Maybe someone of you has an idea where to find this information - Thanks

For the first question, a paper on Amundsen glaciers is doi: 10.1002/2014GL060140 I have referred to it before. This thread contains answers to most of the other questions.

" a change in grounding line retreat velocity is due to local effects (e.g. bumps in the glacier's way) or general conditions (e.g. warming of the deeper ocean water) "

Yes. The shelves are pinned on high points and the grounding line on peaks in the beds.

" whether there is a correlation between the speeding-up of each glacier's flow downstream and the grounding line position. "

This is more interesting. While grounding line retreat is more sensitive to warm water intrusion and bed slopes, the overall flow velocity is more complex, controlled by SMB, bed and surface slope, beg properties and basal hydrology. For example there is a presentation by alley that shows that PIG has a traffic jam upstream where several tributaries come together. When grounding line reaches that point ...